Burner control system

ABSTRACT

Disclosed is a fail safe burner control system with a valve controller for operating a burner, a flame rectification flame detector and a spark igniter apparatus. A first electronic switch opens the valve and starts the igniter. Upon the occurrence of flame, a second electronic switch, in response to the rectification detector, disables the first switch thus stopping the spark, but holds the valve open as long as flame is sensed. If flame is lost, the first switch is enabled automatically. In the event of failure to reignite after a loss of flame, the continued operation of the sparking igniter causes a circuit breaker to lock out the system.

United States Patent 1191 Walbridge 5] Aug. 27, 1974 BURNER CONTROL SYSTEM 3,726,630 4/1973 P0115 431/78 75 l t L H. Walbri e Ashland, 1 men or dg Primary ExaminerEdward G. Favors Attorney, Agent, or Firm-John E. Toupal [73] Assignee: Walter Kidde & Company, Inc.,

Clifton, NJ. I ABSTRACT [22] Flled: 1972 Disclosed is a fail safe burner control system with a [21] Appl. No.: 306,591 valve controller for operating a burner, a flame rectification flame detector and a spark igniter apparatus. A first electronic switch opens the valve and starts the igniten Upon the occurrence of flame, a Second elec 58] Fieid 67 70 tronic switch, in response to the rectification detector, 0 c 1 6 disables the first switch thus stopping the spark, but holds the valve open as long as flame is sensed. If 56 R f Cted flame is lost, the first switch is enabled automatically. 1 e etences, I In the event of failure to reignite after a lossof flame, UNITEDSTATES PATENTS I the continued operation of the sparking igniter causes 3,377,125 4/ 1968 Zielinski .L 431/74' a circuit breaker to lock out the system. 3,384,440 5/1968 Mayer 431/66 a 3,649,156 3 1972 Conner ..431/7s 26 Clams, 3 Drawmg Flgures 39 23 RES E L 22 i I M 37 1 DELAY 1 FIRST 1 SECOND S FLAME TIMER SCR' s cR SENSOR L 1 1 2s r 33 38 32 P 1 1 1 SHUT \GNITION FUEL DOWN TIMER I GNITER VALVE TIM ER 2e LOCK 21 3.832.123 saw aor a PATENIEUmnzmu BURNER CONTROL SYSTEM BACKGROUND OF THE INVENTION The invention relates to burner control systems and, more particularly, to fail safe burner control systems.

Extensive efforts have been directed toward the improvement of control systems for fuel burners such as gas and oil burners and the like. Increased system safety and reliability have been primary objectives of such efforts. These objectives, however, generally conflict with an obvious desire to limit the cost and physical size of the systems. Thus system complexity is an important consideration.

Such systems often ignite the fuel with a spark igniter. Interest has recently been directed toward systems that extinguish the spark after ignition to eliminate radio frequency interference. However, circuits to extinguish the spark have greatly added to the complexity of the control circuit. This is particularly true since it is required that if flame. is lost for any reason, the system must respond in one of two vways. Either the valve must be closed to stop the flow of fuel, or as is preferable if heat is still required, the ignition apparatus must be activated in an effort to reestablish flame.

In addition, most burner systems must employ fuel supply valves that are controlled by flame sensing mechanisms which automatically interrupt fuel flow in response to a predetermined loss of flame condition. In accordance with the above requirements, circuits have been designed wherein the spark apparatus is responsive to the flame sensor so that when flame is detected the igniter is stopped and uponloss of flame the igniter is activated to reestablish flame. A difficulty encountered with these circuits is their complexity; for example, often a plurality of feedback loops, or the like, is used. A danger in having such a complex system is that failure ofone or more circuit components can cause an unsafe condition as, for example, a situation in which the valve remains open but the ignition apparatus isnot activated. An explosive amount of fuel may thereby enter the atmosphere. Many conventional circuits provide a capacitor that is charged by the sensor when flame is present and a valve that opens when the charge on the capacitor exceeds a predeterminee minimum.

To initiate operatiomthe capacitor is precharged to open the valve and is kept charged by the sensor if flame is achived. If no flame is achieved before the capacitor becomes discharged, the valve closes and the system shuts down. The unsafe condition can occur in this circuit, for example, if flame is lost or never established, but a malfunction in the precharging circuit keeps the capacitor'charged and thus the valve open. This is a result of having to fool" the system by precharging. I g

The object of this invention, therefore, is to provide a burner control system that automatically activates the system for a fuel burner comprising a valve apparatus that controls the flow of fuel to the burner, and a spark ignition apparatus. A flame detector circuit is also included. A first electronic switching apparatus is energizable to open the valve and start the ignition apparatus. A second electronic switching apparatus is coupled to the flame detector circuit, and, in response to a signal indicating the presence of a flame, maintains the valve in its open position and, through an ignition interruptions apparatus, inhibits the first switch apparatus. As long as power is supplied to the control system, the first switch is automatically enabled in the absence of a signal from the ignition interruption apparatus. Consequently, a control circuit is provided that automatically reestablishes flame in the event of a loss thereof by immediately enabling the first switch to start the ignition apparatus and maintain the valve in an open position. An advantage of providing two switches, each of which is capable of maintaining the valve in an open position, is that'many of the dangers experienced with precharging circuits, such as maintaining the valve in an open position without flame, are eliminated. A separate three terminal solid state element, for example a silicon controlled rectifier, forms the active element of each of the electronic switching apparatus. A circuit v utilizing three terminal devices is rendered significantly more failsafe than conventional relay controlled circuitry. This is so because conventional circuits utilize multiple pole relays with contacts controlling a plurality of separate switching circuits. Thus the valve and the ignition apparatus are generally controlled by separate circuits; Therefore a component failure in either circuit, or a pair of relay contacts sticking together can igniter to reestablish flame in the event of a loss thereof. However, it is desired that if a failure to establish flame occurs, either initially or after a loss of flame, the valve be closed after a predetermined time. It is further desired that the system be rendered fail safe, that is, malfunction of any component or group of components shall not lead to an unsafe condition.

SUMMARY OF THE INVENTION The invention is characterized by a burner control cause the valve to remain open without activating the ignition apparatuslHowever, such an event is less likely to occur in the subject control system inasmuch as SCRs function as if they were two terminal SPST devices. therefore, the single SCR in the first switch both energizes the spark apparatus and opens the valve with the equivalent of only one pair of contacts. Thus, for example, it is unlikely that one of these functions will .be performed in response to the first SCR without the other function being performed also.

A feature of the invention is the inclusion of a delay timer apparatus in the first switch inconjunction with a flame detector system. The timer prevents the enabling of the first electronic switching apparatus until the timer has timed out in a predetermined delay time. The flame rectification detector fires the SCR in the second electronic switch apparatus once during each cycle of the ac. voltage supplied to power the burner control system. Pulses caused by the conduction of the SCR in the second switch are coupled to the delay timer by a periodic reset apparatus and each pulse resets the delay timer. Since the delaytime is selected to be longer than the period of the alternating supply voltage the delay timer can never time out if the flame rectification detector circuit is sensing flame. However, in the event that no flame is sensed, the delay timer is no longer reset and it quickly times out. Thereafter the first electronic switch is enabled and the ignition apparatus is energized. Thus it is seen that the ignition apparatus is energizedin an effort to reestablish flame when flame is lost, and furthermore that the ignition apparatus is deenergized upon establishing flame.

Another feature of the invention is the inclusion of a circuit to lock out after a predetermined period of ignition unless flame is sensed. This is important inasmuch as the valve is open while the first electronic switch is activated so that if there is a malfunction in the ignition apparatus, the valve is releasing fuel into the atmosphere with no change of the ignition thereof. Power is supplied to the burner control system through the circuit breaker. A wave shaping control circuit within the burner system causes the silicon controlled rectifier in the first electronic switch to conduct for a substantially longer duty cycle than is exhibited by the SCR in the second electronic switch. Furthermore, operation of the first electronic switch causes power to be drawn directly from the power supply, but as will be explained below, normal operation of the second switch does not. The result of this is that the normal operation of the second electronic switch puts no significant strain on the thermal circuit breaker, but continued operation of the first electronic switch, drawing power during its longer duty cycle, will cause actuation of the circuit breaker after a preselected period of time. Thus, operation of the first electronic switch continuously for seconds, for example, will cause actuation of the thermal circuit breaker.

DESCRIPTION OF THE DRAWINGS These and other features and objects of the present invention will become more apparent upon a perusal of the following description taken in conjunction with the accompanying drawings wherein:

FIG. I is an operational diagram of a preferred burner control system;

FIG. 2 is a schematic diagram of the preferred system; and

FIG. 3 shows various wave forms at different points within the circuit shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1 there is an operational diagram of a preferred burner control system 21. It should be understood that the diagram of FIG. 1 is not a con ventional block diagram. For example, not all of the blocks depicted in FIG. 1 correspond to an easily descernible portion of the circuit shown in FIG. 2, and the lines coupling the blocks in FIG. 1 may indicate either electrical or mechanical coupling. It is however, felt that an understanding of the diagram shown in FIG. 1 will simplify comprehension of the operation of the circuit shown in FIG. 2. The system 21 is powered by an ac. power source (not shown). When power is applied, a delay time 22 that is part of a first electronic switch 23 begins to time out in a predetermined delay time that is longer than one cycle of the ac. supply current. The delay timer 22 enables a first silicon controlled rectifier 24 through a line 25. The first silicon control rectifier 24 fires once during each cycle of the ac. current as long as a signal remains on the line 25. In addition, the signal on the line 25 is carried to a shut down timer 26 that begins timing out in a preselected shut down time when enabled. If the shut down timer 26 times out, a signal delivered on a line 27 to a lock out apparatus 28 causes the system 21 to lock out. Firing of the first SCR 24 produces a signal on a line 29 that performs three functions. An igniter 31 is energized in response to a signal on the line 29 and a fuel valve 32 is opened in response thereto. Thus when the first SCR 24 fires, fuel is supplied to a burner (not shown) and the igniter 31 seeks to ignite the fuel. Simultaneously, an ignition timer 33 begins running in response to the signals on the line 29. If the ignition timer 33 times out indicating that the first SCR 24 has been firing for a preselected period of time, a signal on a line 34 is delivered to the lock out apparatus 28 thus locking out the system 21. As was pointedout above, the first SCR 24 fires whenever there is a signal on the line 25. Thus the presence of a signal on the line 25 starts operation of both the shut down timer 26 and the ignition timer 33. When either timer 26 or 33 times out, the lock out apparatus 28 is activated. Thus the two timers 26 and 33 are both ignition timers and the provision of two separate timers is a safety feature. Disposed near the burner is a flame sensor 35 that fires a second SCR through a line 37 once each cycle of ac. power when flame is sensed. When the signal on the line 37 is delivered to the second SCR 36, it fires producing pulses on a line 38 that maintain the valve 32 in an open position and resets the delay timer 22 through a periodic reset line 39.

During operation of the system 21 power is applied and the delay timer 22 times out in the delay time of greater than one cycle of the ac. supply voltage. When the delay timer 22 has timed out, the first SCR 24 begins to tire and the shut down timer 26 and the ignition timer 33 begin to run. Also in response to the firing of the first SCR 24, the igniter 31and fuel valve 32 are energized. Under normal circumstances flame will be established before either the shut down timer 26 or the ignition timer 33 has'timedout. In that event, the flame sensor 35 begins firing the secondSCR 36 which maintains the valve 32 in an open position and, upon firing once eachcycle of the supply voltage, resets the delay timer 22 through the periodic reset line 39. Recalling that the delay time is greater than the period of the ac. supply voltage, it is seen that the delay timer 32 is prevented from timing out while the second SCR 36 is firing. Thus it is seen that as long as flame is sensed by the flame sensor 35 the shut down timer 26 and the first SCR 24 are inoperable. If flame is lost, the second SCR 36 ceases firing the delay timer 22 soon times out thus causing the first SCR 24 to resume firing. Consequently the effect of a loss of flame is that the system behaves as it does when initially energized. Thus if flame is reestablished the second SCR 36 begins to fire again and the'first SCR 24 is inactivated and the shut down timer 26 is periodically reset.

If flame is not established initially, or following an effort to reignite, the system 21 is locked out upon the timing out of either the shut down timer or the ignition timer 33.

Referring now to FIG. 2 there is a schematic diagram of the burner control system 21. Portions of the circuit corresponding to the blocks in FIG. 1 have been pointed out with similar reference numerals where possible. A hot" line 41 in an ac. power supply in connected to a buss 42 by a switch 43 such as, for example, a thermostat. A grounded line 44 is connected to a lock out thermal circuit breaker 45, that is part of a power input apparatus, so that the current flowing through the line 44 passes through an energy accumulating bimetalic strip member 33. A threshold member 34 in the circuit breaker 45 separates switch deactivator lock out contacts 28 in the event of a circuit breaker overload as evidenced by an excessive amount of heat building up in the bimetalic member 33. The heat energy in the bimetalic member 33 is supplied by heating caused by current flowing therethrough and the surface of the bimetalic element 33 radiates heat from the strip 33 to the atmosphere and thus comprises an energy leakage system. Because energy is radiated by the surface of the bimetalic strip 33, the circuit breaker 45 will not respond to energy supplied thereto at a low rate. The circuit breaker 45 connects the grounded line 44 to a junction 46. The power supplied on the lines 41 and 44 is alternating current and the term positive half cycle means that half of the cycle of the alterntating current in which the line 44 is positive with respect to the line 41. It will be appreciated that the absolute potential on the grounded line 44 does not change and that changes in voltage refer only to relative values with respect to power line 45. I

Controlled by the system is a fuel burner 47 that is grounded and is supplied with fuel through a line 48 in response to a valve control apparatus 32 including a valve control relay coil 49 that is shunted by a capacitor 51. When the coil 49 is energized the valve is opened. A diode 52 couples the coil 49 and capacitor 51 combination across a resistor 53. One end of the coil 49 is connected to the common buss 42 along with one end of the capacitor 51 and the resistor 53. The other end of the resistor 53 is connected in series with a capacitor 54 and thence another resistor 55. At a junction 56 the resistor 55 is connected to a spark capacitor 57, the other end which is connected to the buss 42. The junction is also connected to the anode of the second SCR 36 by a diode 106 and a resistor 89. During positive half cycles of the input voltage, the capacitors 54 and 57 charge through the spark igniter apparatus 31 that includes a resistor 58 and diode 59 in series with a primary winding 61 of a spark transformer 62. Current flow in the above described spark circuit is prevented during negative half cycles of the supply voltage by the diode 59. Also included within the ignition apparatus 31 is a secondary winding of the transformer 62 with two spark electrodes 64 and 65 connected thereto. The magnitude of the charging current is insufficient to cause sparking between the electrodes. The flame rectification flame detector aparatus 35 includes a resistor 66 connected between the electrode 65 and aflame rectification capacitor 67. The other terminal of the capacitor 67 is connected to the buss 42. Shunting the capacitor 67 is a resistor 68 and connected to a parallel combination of a capacitor 69 and complementary silicon controlled rectifier 71 by another resistor 72. Two capacitors 73 and 74 connected in series and joined at a junction 75 shunt the complimentary silcon con trolled rectifier 71. A resistive voltage divider including a resistor 76 and a resistor 77 spanning from the junction 46 to the buss 42 supplies current to the gate 78 of the complementary silicon controlled rectifier 71.

The first electronic switch apparatus 23 including the first SCR 24 is made to conduct by applying a voltage to a junction 81 that powers a voltage divider control including two resistors 82 and 83 that supply current to the gate 84 of the SCR 24. The second electronic switch apparatus 85 including the second SCR 36 receives power from the junction 46 through a resistor 86, a diode 87, a inhibit diode 88 and another resistor 89. The preceding circuit is a cut off control circuit 90. The gate 91 of the SCR 36 is connected to the junction by the line 37 and to the buss 42 by a resistor 92.

The delay timer clamping capacitor 22 connects a periodic reset line 94 to the buss 42. The cut off circuit 90 and the delay timer clamping capacitor 22 are part of an ignition interruption apparatus that deenergizes the ignition apparatus 31 upon the sensing of a flame by the flame sensor 35 as will be described more fully below.

The shut down timer 26 includes an energy accumulator capacitor 95 and a leakage resistor 96 in series and connected between the line 94 and the buss 42. A junction 97 between capacitor 95 and the resistor 96 is coupled to the gate 98 of a shut down silicon controlled rectifier 99 by a neon bulb 101. A capacitor and a resistor are connected in parallel between the gate 98 and the cathode of the SCR 99 and the anode is coupled to the line 94 by a resistor 105. Any energy absorbed by the capacitor is leaked off through the leakage resistor 96 when the second SCR 36 is firing as described below. When the SCR 99 fires, it acts as a controlling apparatus for the first SCR 24 so that the SCR 24 conducts. A control circuit 102 including a capacitor 103 and a neon bulb 104 supplies current to the gate 84' of the first SCR 24 through the junction 81. The capacitor is charged through a resistor 105.

Referring now to FIG. 30' there are shown charging curves for the capacitors 103, 22 and 95. It is to be understood that no specific time constants are shown because theexact time constants are less important than the relationship among the three charging time constants. It should be further understood that the curves shown are for charging each capacitor disregarding the effect of the other capacitors. Specifically, the clamping action of the capacitor 22 on the capacitor 95 is ignored in FIG. 3a. The time t represents approximately one cycle of the alternating supply voltage. Thus it is seen. by a curve 111 that in this example the capacitor 103 is nearly fully charged after one cycle. The delay capacitor 22, as represented by a curve 112, requires several cycles to obtain a substantial charge and the capacitor 95 requires many cycles as shown by a curve 113. The capacitor 95 could, for example, take approximately 10 seconds to charge.

During operation of the system 21 a.c. power is supplied and during the positive half cycles thereof current flows through the circuit breaker 45, the diode 59 and the primary winding 61 to charge the capacitors 54 and 57, which nearly fully charge during one half cycle. In addition, current flows through the resistor 86 to the capacitors 103, 22 and 95. During negative half cycles of power, the capacitor 103 is bypassed by a diode and thus does discharge. The diode 87 prevents discharge of the capacitors 22 and.95 in the negative half cycles except through the SCR 36. Two paths of discharge are available for the capacitors 54 and 57. One path is through the primary winding 61 and then through the SCR 24. The second is through the resistor 89 and then through the second SCR 36.

To more fully understand the operation of the system 21, reference should be made to FIGS. 3(b) (f). A sine wave form 121 shown in FIG. (3b) represents the alternating current power supplied to the system 21 and is used to establish a time scale for FIGS. 3(c) (f). A curve 122 in FIG. 3(0) showsthe delay capacitor 22. A small amount of charge is gained during each positive half cycle of the sine wave 121 and the charge on the capacitor 22 remains constant during negative half cycles. The charge on the capacitor 103 is shown by a wave form 123 in FIG. 3(d). The capacitor 103 can substantially charge during one positive half cycle of the sine wave 121. However, during the positive half cycles the diode 87 is forward biased and thus is conductive so that the charging of the capacitor 103 is initially delayed by the clamping of the delay clamping capacitor 22 as shown in FIGS. 3(a) and (d).

After several cycles the capacitor 22 approaches full charge each cycle and allows the capacitor 103 to fire the neon bulb 104. Firing occurs at near the peak of the positive half cycle of the sine wave 121 as shown at the points 124 in FIG. 3(d). Discharge of the capacitor then proceeds through the bulb 104 and the wave shaping resistors 82 and 83 to supply current that causes the first SCR 24 to conduct. The resistor 82 length the discharge period of the capacitor 103 so as to prolong the current input to the gate 84 and thereby the conduction period of the SCR 24. After an initial period of discharge, the bulb 104 stops conducting and the discharge proceeds as shown by the curved portion 125 of the wave form 123. Thus the first SCR 24 conducts during half of the positive half cycle as shown by a wave form 126 in FIG. 3(e). Inasmuch as the capacitors 54 and 57 absorb substantially a full charge during each positive half cycle of the wave form 121, they supply a substantial current to the primary winding 61. as they discharge through the SCR 24. This current creates sufficient power in the secondary winding 63 to cause a spark between the electrodes 64 and 65. In addition, the discharge of the capacitor 54 creates a current through the resistor 53 and a voltage drop thereacross as indicated in FIG. 2. This voltage drop forward biases the blocking diode 53 and thus activates the relay coil 49. It should be noted that even if the diode 52 were to become shorted, the valve would not open when the capacitor 54 is charging. The current flow then is to low due to the resistor 58. Generation of a large enough voltage across the resistor 53 requires storing a charge in the capacitor 54 and drawing it out in a rapid surge that bypasses the resistor 58. In addition, the capacitor 51 stores a sufficient charge to maintain the valve open until the following positive half cycle. Thus gas is released from the burner 47 and the ignition apparatus 31 sparks when the SCR 24 fires.

When flame is achieved at the burner 47 current is conducted between the burner and the electrode 65 in accordance with the flame rectification phenomena, thereby charging the capacitor 67 to the. polarity indicated in FIG. 2. This charge is filtered and impressed across the capacitor 73 by the resistors 68 and 72 and the capacitor 69. The capacitors 73 and 74 are connected in series, and the combination is in parallel with the capacitor 69 and thus they are charged with the polarity indicated in FIG. 2. Note that the capacitor 74 is charged to a lower level than the capacitor 73 due to the drain of the resistor 92., In order for the complementary SCR 71 to conduct, the gate 78 thereof must receive current from the anode. This situation occurs during each negative half cycle of the sine wave 121 due to the resistors 76 and 77. In order for the second SCR 36 to fire, itmust pass current from the gate 91 1 73, the complementary SCR 71 is fired at the negative going crossovers 127 of the wave form 121. When the complementary SCR 71 fires, it effectively connects the negatively charged terminal of the capacitor 73 to the buss 42, thus discharging the capacitor 73 through the gate 91 of the second SCR 36. Consequently, when there is sufficient charge on the capacitor 73, the complementary SCR 71 and the second SCR 36 both fire on the negative going crossovers 127. The firing cycle of the second SCR 36 is shown by a wave form 128 in FIG. 30). Comparing FIGS. 3(e) and (f) it is noted that the first firing of the second 36 occurs precisely at the conclusion of a conducting cycle of the first SCR 24. Thus the capacitors 54 and 57 have been previously discharged by the first SCR 24. However, as shown by FIG. 3(c), the first firing and each subsequent firing of the second SCR 36 discharges the delay capacitor 22 through the inhibit diode 88. Inasmuch as the second SCR 36 fires every cycle if flameis sensed, the delay capacitor 22 requires several cycles before a sufficient charge can be built up to permit the first SCR 24 to fire, the first SCR 24 does not fire when the second SCR 36 is firing.

The capacitor 54 and 57 each continueto absorb a full charge during each positive half cycle of the wave form 121. However, discharge is now through the second SCR 36. Thus the voltage is still produced across the resistor 53 to maintain the valve in open position, however, the primary winding 61 of the transformer 62 is bypassed and thus the spark ignition apparatus 31 is deenergized' and the spark is extinguished. This mode of operation continues as long as flame is sensed.

Note the constant time lines T, and T in FIGS. 3(b) to (f). It will be observed that the values represented by the wave forms are identical at each line. Thus it will be appreciated that if thefiring of the second SCR 36 indicated by a pulse 129 on the wave form 128 were not to occur, the situation would be precisely as it was at the time T The-pulse 129 will not occur if flame is lost because the flame rectification capacitor 67 then becomes discharged. Thus it will be appreciated that if flame is lost, the system 21 automatically recycles to try for reignition.

Consider the system operation in the event of a failure to establish flame. When the delay capacitor 22 becomes charged after a few cycles, the energy absorbing capacitor begins to charge. After 10 seconds the capacitor 95 has stored a sufficient charge to fire the neon bulb 101 which causes the SCR 99 to conduct establish ignition. The small lobe 133 is due to the charging of the capacitors 54 and 57. The large conductingportion 134 corresponds in shape to the firing of the first SCR 24 as shown in FIG. 3(a) and indeed represents the firing of the first SCR. The first SCR 24 conducts such a large current because it fires during the positive half cycles of the supply voltage and thus a current path is established from the junction 46 through the resistor 58, the diode 59 and the SCR 24 to the buss 42. Consequently, a strain is put on the thermal lock out circuit breaker 45 during the long duty cycle of the first SCR 24. If the shut down SCR 99 fails to fire for any reason, the continued firing of the first SCR 24 in an effort for ignition will cause the circuit to lock out after approximately 15 seconds. Conversely, if the control circuit 102 malfunctions such that the first SCR 24 fires late in the positive half cycle, the large lobe 134 will not occur and continued operation of the SCR will not put a strain on the circuit breaker 45, the valve may be held in an open position. In that event, conduction of the SCR 99 is relied on to cause lock out. The provision of two possible methods for lock out is beneficial as a safety feature.

FIG. 30') shows a wave form indicating current passed by the lock out circuit breaker 45 when the second SCR is firing. Small lobes 135 correspond to the charging of the capacitors shown by the lobes 133.

There is no large current surge when the SCR 36 fires at the negative going crossovers because no substantial power is being applied to the lines 41 and 44 and what power is applied during the negative half cycles reverse biases the second SCR after the discharge of the capacitors 54 and 57. Thus the only power conductedby the second SC R 36 is the discharge of the capacitors 22, 54 and 57. Consequently, continued operation of the second SCR 36 will not cause the activation of the lock out circuit breaker 45. However, should the second SCR 36 becomes shorted or leaky, power will pass therethrough during the positive half cycles, causing overloading of the circuit breaker 45.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. For example, the preignition timing may be extended to provide substantial purge time and the ignition timer may be adapted to closing the valve without opening the circuit breaker using conventional circuitry. It is therefore, to be understood that'within the scope of the appended claims the invention can be practised otherwise than as specifically described.

What is claimed is:

1. A burner control system comprising: 1

valve means for controlling the flow of fuel to a burner; ignition means energizable to ignite fuel emanating from the burner; flame detector means for detecting the presence of flame at the burner; first electronic switch means enableable to both activate said valve means to initiate fuel flow to the burner and energize said ignition means.

second electronic switch means for maintaining said valve means in a condition wherein fuel is supplied to the'hurncr in response to signals from said flame. detector means; and

ignition interruption means or deenergizing said ignition means.

.2. A system according to claim 1 wherein said first electronic switch means and said second electronic switch means comprise solidstate switching elements.

3. A system according to claim 1 wherein said ignition interruption means comprises cut-offmeans responsive to said second electronic switch means for disenabling said first electronic switch meanswhen flame is sensed by said flame detector means.

4. A system according to claim 3 wherein said flame detector means comprises electrode means supplied with alternating electric current and disposed to be bathed by the flame and direct current detection means for detecting the flow of rectified current through the flame.

5. A system according to claim 3 wherein said ignition interruption means comprises delay timer means for disenabling said first switch means until said delay timer means has timed out in a predetermined delay time, and wherein said cut-off means comprises inhibit means for deactivating said delay timer means and thereby disenabling said first switch means.

6. A system according to claim 5 wherein said cut-off means comprises periodic reset means for periodically resetting said delay timer at intervals shorter than said predetermined delay time when the presence of a flame is sensed.

7. A system according to claim 6 wherein said delay timer comprises clamping means responsive to said periodic reset means for clamping the power supplied to said first switch means.

8. A system according to claim 7 wherein said clamping means comprises a clamping capacitor that is periodically discharged by said periodic reset means.

9. A system according to claim 6 comprising ignition timer means for timing the operation of said ignition means and including power input means for supplying power to said burner control system and further comprising switch deactivator means for disenabling said first electronic switch means in response to timing out of said ignition timer means in a predetermined period of time.-

10. A system according to claim 9 wherein said switch deactivator means comprises energy accumula tion means for normally receiving energy at a first rate through said first electronic switch means and comprising threshold means for disconnecting power from said control system after the accumulation of a predetermined amount of energy.

11. A system according to claim 10 wherein said switch deactivator means comprises lock out thermal circuit breaker means.

12. A system according to claim 10 wherein said energy accumulation means comprises leakage means for 14. A system according to claim 13 comprising control circuit means for causing said first controlled rectifier to conduct for a longer duty cycle than that of said second silicon controlled rectifier.

' 15. A system according to claim 12 comprising ac. power supply means for rendering said first electronic switch means conductive during one half cycle of the ac. power supplied and wherein said second electronic switch means is conductive during the alternate half cycle of the ac. power supplied.

16. A system according to claim 15 comprising control circuit means for supplying energy to said energy accumulation means in the event of conduction by said second electronic switch means during said half cycle of each cycle and preventing the flow of any substantial amount of energy through said second electronic switch means to said energy accumulation means during said alternate half cycle.

17. A system according to claim 16 wherein said first electronic switch means comprises a first silicon controlled rectifier and said second electronic switch means comprises a second silicon controlled rectifier.

18. A system according to claim 17 wherein said ignition means comprises spark ignition means.

19. A system according to claim 6 comprising lock out means adapted to transmit power to said system during normal periodic energization of said second electronic switch means and to lock out said system following normal periodic energization of said first electronic switch for a predetermined period of time.

20. A system according to claim 19 wherein said lock out means comprises a thermal circuit breaker.

21. A system according to claim 1 wherein said valve means comprises electromagnetic means for controlling the flow of fuel, said first electronic switch is connected to transmit electrical power to said electromagnet means for opening said valve means, and said sec ond electronic switch means is connected to transmit electrical power to said electromagnetic means for maintaining said valve means in open position in response to signals from said flame detector means.

22. A system according to claim 1 comprising ignition timer means for timing the operation of said ignition means and including power input means for supplying power to said burner control system and further comprising switch deactivator means for disenabling said first electronic switch means in response to timing out of said ignition timer means in a predetermined period of time.

23. A system according to claim 22 wherein said switch deactivator means comprises energy accumulation means for normally receiving energy at a first rate through said first electronic switch means and comprising threshold means for disconnecting power from said control system after the accumulation of a predetermined amount of energy.

'24. A system according to claim 23 wherein said energy accumulation means comprises leakage means for dissipating energy stored therein and wherein said second electronic switch means normally transmits energy to said energy accumulation means at a second rate lower than the said first rate, and said leakage means renders said lock out means nonresponsive to said second rate.

25. A system according to claim 1 comprising lock out means adapted to transmit power to said system during normal periodic energization of said second electronic switch means and to lock out said system following normal-periodic energization of said electronic switch for a predetermined period of time.

26. A system according to claim ll whereinsaid first electronic switch means, said ignition means and said valve means are interconnected such that activation of said valve means by said first switch requires energization of said ignition means. 

1. A burner control system comprising: valve means for cOntrolling the flow of fuel to a burner; ignition means energizable to ignite fuel emanating from the burner; flame detector means for detecting the presence of flame at the burner; first electronic switch means enableable to both activate said valve means to initiate fuel flow to the burner and energize said ignition means. second electronic switch means for maintaining said valve means in a condition wherein fuel is supplied to the burner in response to signals from said flame detector means; and ignition interruption means or deenergizing said ignition means.
 2. A system according to claim 1 wherein said first electronic switch means and said second electronic switch means comprise solid-state switching elements.
 3. A system according to claim 1 wherein said ignition interruption means comprises cut-off means responsive to said second electronic switch means for disenabling said first electronic switch means when flame is sensed by said flame detector means.
 4. A system according to claim 3 wherein said flame detector means comprises electrode means supplied with alternating electric current and disposed to be bathed by the flame and direct current detection means for detecting the flow of rectified current through the flame.
 5. A system according to claim 3 wherein said ignition interruption means comprises delay timer means for disenabling said first switch means until said delay timer means has timed out in a predetermined delay time, and wherein said cut-off means comprises inhibit means for deactivating said delay timer means and thereby disenabling said first switch means.
 6. A system according to claim 5 wherein said cut-off means comprises periodic reset means for periodically resetting said delay timer at intervals shorter than said predetermined delay time when the presence of a flame is sensed.
 7. A system according to claim 6 wherein said delay timer comprises clamping means responsive to said periodic reset means for clamping the power supplied to said first switch means.
 8. A system according to claim 7 wherein said clamping means comprises a clamping capacitor that is periodically discharged by said periodic reset means.
 9. A system according to claim 6 comprising ignition timer means for timing the operation of said ignition means and including power input means for supplying power to said burner control system and further comprising switch deactivator means for disenabling said first electronic switch means in response to timing out of said ignition timer means in a predetermined period of time.
 10. A system according to claim 9 wherein said switch deactivator means comprises energy accumulation means for normally receiving energy at a first rate through said first electronic switch means and comprising threshold means for disconnecting power from said control system after the accumulation of a predetermined amount of energy.
 11. A system according to claim 10 wherein said switch deactivator means comprises lock out thermal circuit breaker means.
 12. A system according to claim 10 wherein said energy accumulation means comprises leakage means for dissipating energy stored therein and wherein said second electronic switch means normally transmits energy to said energy accumulation means at a second rate lower than the said first rate, and said leakage means renders said lock out means non-responsive to said second rate.
 13. A system according to claim 12 wherein said first electronic switch means comprises a first silicon controlled rectifier and said second electronic switch means comprises a second silicon controlled rectifier.
 14. A system according to claim 13 comprising control circuit means for causing said first controlled rectifier to conduct for a longer duty cycle than that of said second silicon controlled rectifier.
 15. A system according to claim 12 comprising a.c. power supply means for rendering said first electronic switch means conductive during one half cycle of the a.c. power supplied and wherein said second electronic switch means is conductive during the alternate half cycle of the a.c. power supplied.
 16. A system according to claim 15 comprising control circuit means for supplying energy to said energy accumulation means in the event of conduction by said second electronic switch means during said half cycle of each cycle and preventing the flow of any substantial amount of energy through said second electronic switch means to said energy accumulation means during said alternate half cycle.
 17. A system according to claim 16 wherein said first electronic switch means comprises a first silicon controlled rectifier and said second electronic switch means comprises a second silicon controlled rectifier.
 18. A system according to claim 17 wherein said ignition means comprises spark ignition means.
 19. A system according to claim 6 comprising lock out means adapted to transmit power to said system during normal periodic energization of said second electronic switch means and to lock out said system following normal periodic energization of said first electronic switch for a predetermined period of time.
 20. A system according to claim 19 wherein said lock out means comprises a thermal circuit breaker.
 21. A system according to claim 1 wherein said valve means comprises electromagnetic means for controlling the flow of fuel, said first electronic switch is connected to transmit electrical power to said electromagnet means for opening said valve means, and said second electronic switch means is connected to transmit electrical power to said electromagnetic means for maintaining said valve means in open position in response to signals from said flame detector means.
 22. A system according to claim 1 comprising ignition timer means for timing the operation of said ignition means and including power input means for supplying power to said burner control system and further comprising switch deactivator means for disenabling said first electronic switch means in response to timing out of said ignition timer means in a predetermined period of time.
 23. A system according to claim 22 wherein said switch deactivator means comprises energy accumulation means for normally receiving energy at a first rate through said first electronic switch means and comprising threshold means for disconnecting power from said control system after the accumulation of a predetermined amount of energy.
 24. A system according to claim 23 wherein said energy accumulation means comprises leakage means for dissipating energy stored therein and wherein said second electronic switch means normally transmits energy to said energy accumulation means at a second rate lower than the said first rate, and said leakage means renders said lock out means nonresponsive to said second rate.
 25. A system according to claim 1 comprising lock out means adapted to transmit power to said system during normal periodic energization of said second electronic switch means and to lock out said system following normal periodic energization of said electronic switch for a predetermined period of time.
 26. A system according to claim 1 wherein said first electronic switch means, said ignition means and said valve means are interconnected such that activation of said valve means by said first switch requires energization of said ignition means. 